1. Field of the Invention
The present invention relates to a charger using radio technology, in particular, to a charger with enhanced charging efficiency.
2. Description of the Related Art
In recent years, so-called RF tags have been under active research and development. The RF tag is a non-contact data carrier that electronically reads and updates package information by using radio technology. In recent years, an RF tag which is combined with a sensor and can read information obtained by the sensor has also been developed. The sensor requires to be equipped with a battery since power supplied by radio transmission is not enough for its operation.
The battery mounted in the RF tag is desirably a charging-type battery in view of the life and usage form of the RF tag. Therefore, there has been proposed a method of charging a battery on a RF tag by using radio power transmission. Generally, in the radio power transmission, a transmission loss is large and therefore, it is a significant issue how the power is transmitted efficiently to charge the battery. A charger using the radio technology is disclosed in, for example, JP-A 2006-314181(KOKAI).